gentleman's agreement

From the idea that a gentleman (a civilized man of good standing) will
honor an agreement he has entered. Earliest documented use: 1821.

USAGE:

"[The teams] are under no requirement to exchange films with a
non-conference team. It is simply a gentleman's agreement when they do."
Jorge Arangure, Jr.; Playing Detective in Finding Films of Opponents;
The New York Times; Sep 4, 2013.